The present invention relates to a rivetless nut plate typically used in the aerospace industry.
Nut plates are used for attaching structural and non-structural components together in both aerospace and commercial applications. There are many different design configurations of nut plates being used today. Two major classes are riveted nut plates and flared rivetless nut plates.
In riveted nut plates, two or more rivets are employed for attaching the body of the nut plates to the workpiece.
Rivetless nut plates are composed fundamentally of three components: a nut element, a holding bracket, and an attachment sleeve. The method of attachment and fixing of these nut plates to workpieces is based on two major features:
(1) The insertion of the sleeve into an aperture of a workpiece by pressing in or pulling down the bracket sleeve assembly into the aperture of the workpiece. Serration/lobe features on the sleeve interfere with walls of the workpiece to provide resistance against the rotation of the nut plate.
(2) After insertion of the sleeve into the aperture of the workpiece, an end of the sleeve is flared against the opposite side of the workpiece providing resistance against push-out forces that are encountered during usage of the nut plate.
One example of this type of rivetless nut plate is embodied in U.S. Pat. No. 4,732,518 which illustrates the insertion of a sleeve inside a workpiece against heavy interference forces and then deformation of the sleeve to produce flaring of the end of the sleeve. The sleeve has a serration/lobe configuration thereon with the serration/lobe configuration being long and tapered such that the serrations/lobes extend into the walls of the workpiece. The tapered feature, length and specific geometry are necessary to make installation possible with the method of installation which was chosen for its application. The main object of the '518 patent with its tapered and extended serration/lobe configuration was to enhance the fatigue life of the workpiece by distributing the load throughout the workpiece and providing expansion due to the insertion of the sleeve into the workpiece, and to cold work the material adjacent the perimeter of the workpiece aperture.
The installation of rivetless nut plates with tooling of present designs, however, such as those discussed in U.S. Pat. No. 4,732,518, is cumbersome, slow, complicated, costly and often non-functional. Also, the hole preparation needs to be precise and requires countersinking or counterboring for flush installation. These shortcomings have limited the usage of rivetless nut plates.
The flaring of rivetless nut plates is also disadvantageous for many reasons, such as complicated tooling, special aperture preparation (such as counter-bore and counter-sink) to achieve flush installation, grip length limitations, the creation of gaps between the sleeve and the walls of the workpiece, and longer length, thus making the rivetless nut plate a relatively heavy component.
Four other rivetless nut plate designs are illustrated in U.S. Pat. Nos. 5,096,349, 5,245,743, 5,405,228 and 5,704,747, which were designed to avoid flaring, but each has disadvantages associated therewith. The design of U.S. Pat. No. 5,704,747, relies on adhesive for attaching the nut plate to the structure. The designs of U.S. Pat. Nos. 5,096,349, 5,245,743 and 5,405,228 do not have adhesives or lobes which are used to fix the nut plate within the structure. These designs rely on heavily cold-worked holes and high interference engagement utilizing a hardened pin as the installation tool to expand the sleeve portion into the structure. The sleeve is smooth and because of high level expansion, the friction of forces created is supposed to retain the nut plate and provide expected mechanical properties. The fundamental purpose for design of this fastener is to enhance mechanical fatigue properties of the joint. The parts for this design are very expensive, installation is costly and cumbersome, and hole preparation needs to be very precise. Thus, the overall cost of this design is very high and application is thereby limited. There are also reports in the field that because of the required heavy expansion the receiving structure can deform beyond acceptable limits, thereby causing damage and rejection of the hole structure.
Thus, there is a need for a rivetless nut plate design which overcomes the disadvantages of the prior art rivetless nut plate designs. The present invention provides for such a rivetless nut plate design.